Kexec (kernel-execute) is a function of the Linux kernel that allows it to act as a bootloader to boot other kernels. Unfortunately, the standard implementation of kexec doesn't work quite right on most ARM devices due to poor driver support for hardware resets. The workaround is kexec-hardboot, a patch set that allows a kernel to be staged in RAM before performing an actual hardware reset through the phone's bootloader. Upon reboot the kexec-supporting kernel will check the magic location in RAM to see if a previously stored kernel is available, and if so, it will transfer execution to that kernel instead of booting itself.

Why use Kexec

It's a second-stage bootloader. The standard Android bootloader only allows two kernels to be installed at once - boot and recovery. This means that if you want a working recovery, you're only allowed one real OS kernel. If you want to dual-boot (or tri-boot or more) you're screwed. Kexec provides an answer to this. By replacing the boot kernel, kexec (with the kexecboot GUI) acts as a "second stage bootloader" allowing you to boot any number of kernels from any available storage devices. For instance, you have kexecboot in your boot partition and you can keep a kernel for Android installed in your Android system partition as well as an Ubuntu kernel and root filesystem on your SD card and be able to switch between Android and Ubuntu at boot time.

Kexecboot is a graphical front-end for kexec. I have modified it to work with the kexec-hardboot patches. It scans all available storage devices for a boot.cfg file in which you define kernels, ramdisks, and kernel commandlines. You control it using volume up and down to move cursor, power to select.

This repository contains all the things you need to build a flashable kexecboot/kexec-hardboot enabled kernel image. It contains the ramdisk with the kexecboot binary and a script to package a flashable zip file. Included as submodules are the kernel source itself (kexec-hardboot branch, required to build the image) and the kexecboot source (optional, not used by default as you must build it using an ARM system, a pre-built binary is included if you don't want to build your own). The kernel source includes a defconfig called kexec_quincyatt_defconfig that sets the required config options for building a kexec-hardboot kernel.

Kexecboot Configuration File

Kexecboot replaces your boot kernel, so when you power up your phone it will go straight to the Kexecboot screen. The issue is now to provide kernels for kexecboot to boot into. This requires some work on your part, as you will have to store the kernel files (zImage and initrd) in a partition and write a configuration file to tell kexecboot where they are. This configuration file may contain multiple kernels, allowing you to have several different kernels available for the same OS or multiple OSes entirely. If you're coming from an Android system that distributes their kernel as a boot.img, you can use the abootimg program to extract it into a separate zImage and initrd.img binary.

The configuration file must be located on the path /boot/boot.cfg. This is relative to whatever partition/disk you are on, so for instance if you're setting up Android to boot from kexecboot, you would put your configuration file in /system/boot/boot.cfg (/data/boot/boot.cfg would work as well). You can also put a boot.cfg file on your SD card as long as you follow the /boot/boot.cfg path. Kexecboot automatically scans all available partitions for a boot.cfg file before it starts and builds a list of all available kernels across all detected boot.cfg files, so you may have Android in your /system partition and Debian on your SD card and both kernel lists will be shown together.

The Note i717 bootloader passes a fairly long string of kernel arguments to the boot kernel. Since kexecboot overrides this for the kexec-booted kernel, you must provide this boot string in your boot.cfg file. Additionally, you may edit or add arguments to the command string here (such as setting console=tty0 instead of the default console=null so you can use the framebuffer console).

I'm not sure how much of that you actually need, but you do need at least some of it because with an empty APPEND= it does not boot. You also do have to put the quotation marks around it or else parsing of one of the options will fail.

My main goal here is native Linux, but if kexec works then you can boot custom Android dev kernels, native Linux kernels, other mobile OS'es, etc. The SGSIII team seems to have found some interesting kexec solutions for the Verizon SGSIII due to its locked bootloader. They've posted a good deal of kexec patches which I'm trying to bring to the Note, including a custom kexec-hardboot option that fully reboots the device into the new kernel (apparently to make sure the radio and such are working).

My main goal here is native Linux, but if kexec works then you can boot custom Android dev kernels, native Linux kernels, other mobile OS'es, etc. The SGSIII team seems to have found some interesting kexec solutions for the Verizon SGSIII due to its locked bootloader. They've posted a good deal of kexec patches which I'm trying to bring to the Note, including a custom kexec-hardboot option that fully reboots the device into the new kernel (apparently to make sure the radio and such are working).

Kexec is actually (if done right) a good way *not* to brick your phone. To run kernels, you usually have to flash them to a restricted boot section of the memory, and if you flash all non-working kernels (to download, recovery, and main) then you have no way to use your phone, as it won't boot up. If you use kexec, your working kernel is safely stored on the boot partition and your development kernels can be wherever, and if it doesn't boot you can just hold down POWER to hard reboot into your good kernel.

The problem is that it doesn't seem to be working, I think I have the kexec support built properly but haven't been able to boot any kernels without it crashing.

Kexec is actually (if done right) a good way *not* to brick your phone. To run kernels, you usually have to flash them to a restricted boot section of the memory, and if you flash all non-working kernels (to download, recovery, and main) then you have no way to use your phone, as it won't boot up. If you use kexec, your working kernel is safely stored on the boot partition and your development kernels can be wherever, and if it doesn't boot you can just hold down POWER to hard reboot into your good kernel.

The problem is that it doesn't seem to be working, I think I have the kexec support built properly but haven't been able to boot any kernels without it crashing.

I know you'll crack it ....
And when you do ....you'll be the galaxy note GOD !!!....LOL
your effort is much appreciated Sir ....even if I'm scared to use it , but will anyway ...lol

So I'm still confused as to why my kexec didn't work. I'm going to build a TouchPad kernel with it enabled and repeat the test on it, since I have a known-good kernel to boot against. I'll let you know how that goes.

2. Fixing fbconsole glitching (framebuffer console displays garbled text that slowly clears up, no clue why...reading /dev/fb0 repeatedly clears up the display immediately and is a dirty hack that works well enough for testing)

XDA Developers was founded by developers, for developers. It is now a valuable resource for people who want to make the most of their mobile devices, from customizing the look and feel to adding new functionality.Are you a developer?